1. Field of the Invention
The present invention relates to a liquid crystal display, and in particular to a supertwist type liquid crystal display with an improved optical compensating plate.
2. Description of the Prior Art
In general, a supertwist type liquid crystal display intrinsically shows a yellow-green or blue color due to the birefrengence of its liquid crystal layer, which can be converted to a bright and clean black-and-white display through color correction using an optical compensator. By this color correction, the display quality is upgraded to a level which enables the liquid crystal display to be utilized for various office automation equipment such as word processors, computers and the like.
Supertwist type liquid crystal displays with color correction of a double-layered type are available and are disclosed, for example, in the Japanese Patent Publications 63-53528 and 63-53529 and in the May Publication of Nitto-Giho in 1989, vol. 27-1, pages 46-53, wherein a coloring made in the first layer acting as a driving panel is color-corrected to provide a display of achromatic color in the second layer acting as an optical compensating panel. This double-layered structure provides a clear black-and-white display with upgraded visibility in comparison with a single-layered supertwist type liquid crystal display. However, since two or more liquid crystal panels are required, this display has problems of being thick in depth and having an increased weight.
In order to solve these problems, the present inventors have developed a thin and light supertwist type liquid crystal display by utilizing an optical retardation film as an optical compensator which is composed of an organic polymer film as disclosed in the Japanese Patent Application 1-262624. However, the retardation film is composed of a uniaxial oriented polymer film which is one of the optical retardation film made of organic polymer film, and since this retardation film is fabricated by extending a polymer film, there arises a difference in its optical nature between the the extending direction of the film and the direction perpendicular to the extension. This then will cause another problem that this supertwist type liquid crystal display utilizing a retardation film has a greater color change in its azimuth direction or elevation direction in comparison with a double-layered supertwist type liquid crystal display utilizing an optical compensating panel which has the same optical nature as that of a liquid crystal display layer (although the optical rotatory directions are reversed therebetween). That is, the supertwist type liquid crystal display utilizing a retardation film has a problem that the optical compensating effect is insufficient, in other words, its viewing angle is narrow in comparison with the double-layered supertwist type liquid crystal display.
Next, optical natures of a uniaxial oriented polymer film and a biaxial oriented polymer film are described below, which are utilized in the present invention as a retardation film.
A uniaxial oriented polymer film is obtained by thermally extending a polymer film in a uniaxial direction after forming the film. A biaxial oriented polymer film is obtained by thermally extending a polymer film in biaxial directions (generally perpendicular) after forming the film.
Referring to the optical nature thereof, when assuming that the refractive indices of the film in the three dimensions are nx (in the extending direction), ny (in a direction perpendicular to the extension) and nz (in the thickness direction), the following relationships are present. However, in a biaxial oriented polymer film, ny is also a refractive index in an extending direction.
In a positive uniaxial oriented polymer film, a relationship is indicated as follows: EQU nx&gt;ny.gtoreq.nz
In a negative uniaxial oriented polymer film, a relationship is indicated as follows: EQU ny.gtoreq.nz&gt;nx
Generally, since a direction of a film having the maximum refractive index is set in a direction of its slow axis (phase lagging axis or S-axis), the slow axis of a positive uniaxial oriented polymer film is coincident with the extending direction of the film, and the slow axis of a negative uniaxial oriented polymer film is coincident with the direction perpendicular to the extension of the film.
In a biaxial oriented polymer film, the extending factors are made so different as to set the biaxially extending directions to be equal, that is, the relationship between the optical refractive indices in the biaxial extending directions is represented by nx=ny. In this case, therefore, the retardation value in the plane of the film is zero, resulting in that the biaxial oriented polymer film does not act as an optical retardation film.
The uniaxial oriented polymer film is utilized as a retardation film for its optical anisotropy. That is, in this arrangement, there is a difference in the refractive indexes of the film between the extended direction of the film and the direction perpendicular to the extension, which is referred to as birefrengence (or double refraction). This birefrengence is a physical phenomenon which occurs in an optically anisotropic substance because the anisotropic substance has different refractive indexes with respect to linear polarized rays having different planes of vibration each other. That is to say, the retardation means a retardation between the phase velocity (or normal velocity) of an ordinary ray and that of an extraordinary ray, wherein an ordinary ray means a ray with a constant phase velocity (refractive index) irrespective of the direction of the light propagating direction in the substance and the extraordinary ray means a ray of which phase velocity (refractive index) is different depending on the propagating direction of light in the substance. Assuming that, the retardation is (R), the refractive index with respect to the extraordinary ray is (ne), the refractive index with respective to the ordinary ray is (no) and that the thickness of the film is (d), the retardation R is represented as follows: EQU R=.DELTA.n.times.d (1)
wherein .DELTA.n=.vertline.ne-no.vertline..
That is, the retardation (.DELTA.n.multidot.d) given by the product of this refractive index anisotropy (.DELTA.n) and the thickness (d) of the film is a physical quantity defining the retardation of light determined when it passes through the film, and the change of the retardation with respect to the elevation angle is different between the extended direction of the film and the direction perpendicular to the extension thereof.
FIG. 1 shows a relationship between the normal direction and the elevation angle (.psi.) in the retardation film having optical anisotropy.
FIG. 2 shows a relationship between the elevation angle .psi. represented by the horizontal axis and the retardation value R (nanometers) represented by the vertical axis in one example of a retardation film having positive optical anisotropy composed of a uniaxial oriented polymer film made from polycarbonate. As shown by .multidot. marks in FIG. 2, the larger the elevation angle .psi., the smaller the retardation value is in the extended direction (the direction of the slow axis, S-axis, referred to as MD hereinafter) of the uniaxial oriented polymer film, while as shown by .smallcircle. marks, the larger the elevation angle, the larger the retardation value increases in the direction (of the fast axis (phase advancing axis), F-axis which is referred to as TD hereinafter) which is perpendicular to the extended direction of the uniaxial oriented polymer film.
In addition, although the case of the retardation film having a negative optical anisotropy is not shown by a figure, the results are opposite to those of the case shown in FIG. 2. That is, when the elevation angle becomes larger, the retardation in the extended direction of the uniaxial oriented polymer film (in the fast axis) decreases, on the other hand, the retardation increases in the direction (slow axis) which is perpendicular to the extended direction of the uniaxial oriented polymer film.
When a retardation film is combined with a liquid crystal display cell, even though the optical compensation is complete in the normal direction, the difference between the retardation of the retardation film and the retardation of the liquid crystal display cell increases as the elevation angle increases, so that the optical compensating relationship is unbalanced. That is, an increased difference between the retardation of the retardation film and that of the liquid crystal display cell will cause light passing through an assembly to have a retardation to result in a colored display. That is, no color compensation is performed and hence narrower viewing angles will result because of deterioration of contrast of the display.
In a supertwist type liquid crystal display utilizing an optical compensator composed of a uniaxial oriented polymer film, there can be obtained a bright and clean display with good visibility in comparison with that of a conventional yellow mode supertwist type liquid crystal display. Therefore, the supertwist type liquid crystal display with an optical compensator composed of a uniaxial oriented polymer film is used as a display unit in many fields such as a display of word-processors or personal computers of lap top type and book type in recent years. In such utilization fields as mentioned above, it is required that the viewing angle of the liquid crystal display is uniformly wide in the vertical and horizontal directions. However, on the other hand, it is required that each display should have a viewing angle suitably wide for its utilization purpose. For example, in a display for use in a telephone, the equipment including a display unit is placed in a horizontal plane in many cases, so that the viewing angle of the display should be especially wide in the frontward (six o'clock) direction. Moreover, in the case of a display for use in an elevator for indicating a number of floor, since the display unit is located in a position higher than the height of human eyes, the viewing angle of the display should be especially wide in the downward direction.
FIG. 3 shows a viewing angle character of a supertwist type liquid crystal display utilizing an optical retardation film composed of a polymer film as an optical compensator which is laminated on one side or both sides of the liquid crystal panel, which is disclosed in the Japanese Patent Application 01-262624 made by the present inventors. This looped line shows a viewing angle character corresponding to a contrast ratio Co (Co.gtoreq.4) when a yellow mode liquid crystal display with a twist angle of 240 degrees is operated with a duty ratio of 1/240.
FIG. 4 shows another viewing angle character of a double-layered supertwist type liquid crystal display utilizing a liquid crystal panel as an optical compensator which is disclosed by the present inventors in the Japanese Patent Publications 63-53528 and 63-53529. This looped line also shows a viewing angle character corresponding to a contrast ratio Co (Co.gtoreq.4) when a monochromatic mode liquid crystal display twisted by 240 degrees is operated with a duty ratio of 1/240. Although the viewing angle thereof is wide to some extent in all azimuths, the viewing angle character in any specific direction is not especially wide.
FIG. 5 shows a viewing angle character of a supertwist type liquid crystal display utilizing an optical retardation film composed of an organic polymer film as an optical compensator laminated on one side or both sides of a liquid crystal panel, which is disclosed in the Japanese Patent Application 02-011156 made by the present inventors. This looped line also shows a viewing angle character corresponding to a contrast ratio Co (Co.gtoreq.4) when a monochromatic mode liquid crystal display twisted by 240 degrees is operated with a duty ratio of 1/240.
FIGS. 3, 4 and 5 show typical viewing angle characters of the conventional liquid crystal displays, and as shown by the figures, the viewing angle in any specific direction could not be significantly extended.
Since the viewing angle of the supertwist type liquid crystal display is mainly dependent on a twist angle of liquid crystal molecules and an orientation axis of the liquid crystal molecules when in operation, the viewing angle in a specific direction can not be extended without deteriorating the quality (performance) of display. Similarly, the double-layered supertwist type liquid crystal display utilizing an optical compensating panel is provided with two liquid crystal panels, and the viewing angle in a specific direction can not be significantly extended.
On the other hand, in a liquid crystal display utilizing a retardation film composed of a uniaxial oriented polymer film, the refractive index in the extended direction of the film is different from that in the direction perpendicular to the extension due to the anisotropy of refractive index peculiar to the uniaxial oriented polymer film. Therefore, the character of the viewing angle can be changed to some extent by selecting an optical fabricating method.
In a uniaxial oriented polymer film having a positive optical anisotropy, for example, a polymer film made from polyvinyl alcohol or a polymer film made from polycarbonate, the extended direction of the film is coincident with the direction of the slow axis and the direction perpendicular to the extension is coincident with the fast axis. Therefore, when viewing in a direction with an elevation angle .psi., the change of the retardation with respect to the elevation angle is different. As shown in FIG. 2, with increment of the elevation angle .psi., the retardation in the direction of the slow axis decreases, while the retardation in the direction of the fast axis increases. Therefore, the character of the viewing angle can be changed to some extent by selecting a suitable optical fabricating method. In this case, however, when combined with a liquid crystal panel, even though the optical compensation is completely performed in the normal direction, with increment of the elevation angle, the difference between the retardation of the uniaxial oriented polymer film and that of the liquid crystal panel becomes large, so that the relationship of the optical compensation is unbalanced. Consequently, there occurs a color change in the achromatizing degree so that the contrast ratio is deteriorated, resulting in a narrow viewing angle. Accordingly, also in this case, the viewing angle character in a specific direction can not be significantly extended.